System for improved memory cell access

ABSTRACT

A voltage booting circuit for booting the switching signal applied to a column access passgate is employed to reduce the voltage drop across the passgate. Reduction of the voltage dropped across the passgate results in faster read and write times and improved noise margin. In one application the booted voltage is used only during a write operation, but not during a read. In another application, the booted voltage is used during both operations.

This application is a continuation of U.S. patent application Ser. No.08/591,758, filed on Jan. 25, 1996.

FIELD OF THE INVENTION

The present invention relates generally to a system for improved memorycell access, and in particular to a booted column access system forrapid memory cell read and write.

BACKGROUND OF THE INVENTION

Improvements in memory device applications result in larger cell arraysand faster memory access requirements. For example, dynamic randomaccess memories (DRAMs) use an array of memory cells, sense amplifiers,drivers and support electronics to manage data read, data write and cellrefresh operations. The memory cells are generally miniature capacitorswhich vary in stored charge according to the voltage written to thecell. Since each memory cell has leakage losses, each cell must berefreshed periodically to prevent loss of the information in each cell.This is called a "writeback" operation and is performed by periodicallyreading each cell and refreshing the cell by internally writing a logicone to the cells storing a one. Additionally, each cell is rewrittenupon a read instruction, since the reads are destructive without theimmediate rewrite.

Static memories (SRAMs) include an array of active memory cells whichmaintain a programmed logic state without the need for refresh.Increasingly larger memory cell arrays add to the latency of cell readand write operations.

Therefore, there is a need in the art for a system for faster memorycell access and refresh. The system should be implemented with thefewest number of modifications to avoid complications to the chiptopology and increased power dissipation.

SUMMARY OF THE INVENTION

The present disclosure describes a memory access system for improvedmemory cell read and write. The present disclosure describes in detail alimited number of environments in which the present invention may bepracticed, however, other applications and environiments exist in whichthe present invention may be practiced.

One environment in which present memory technology may be used is DRAMcircuits. In DRAM architectures, a single write driver and I/O senseamplifier are generally shared among a number of columns in a memoryarray. The sharing is performed by multiplexing the memory array columnsusing a number of passgates which programmably interconnect a pair ofdigit lines to the write driver and I/O sense amplifier. Each passgateisolates the digit lines from the write driver and I/O sense amplifierand each is controlled by a column select signal during a read or writeoperation to a selected memory cell.

In one embodiment of the present memory access system, a voltage boosteris used to increase the column select signal voltage applied to thepassgate to avoid a voltage drop across the passgate during memory cellread and write operations.

In an alternate embodiment, the voltage booster is limited to onlyspecific memory cell operations. For example, the voltage booster isapplied only during a write operation, but not during the readoperation.

Other applications and embodiments are described, such as a staticmemory application, other circuit embodiments, and the use of differentboost voltages, however, these examples are for illustrating the presentmemory access system, and are not intended in an exclusive or limitingsense.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, where like numerals describe like components throughoutthe several views:

FIG. 1 is a functional block diagram of a DRAM circuit;

FIG. 2 is a block diagram of a portion of a DRAM circuit showing theoperation of the passgate in isolating the digit line from the writedriver and I/O sense amplifier;

FIG. 3 is a block diagram of one embodiment of the present memory accesssystem; and

FIG. 4 is a block diagram of one embodiment of the present memory accesssystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiment,references are made to the accompanying drawings which form a parthereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that structural changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims and equivalents thereof.

FIG. 1 is a functional block diagram of a Dynamic Random Access Memory(DRAM), which is one environment in which the present memory accesssystem may be practiced. Those skilled in the art will readily recognizethat other applications of the present memory access system are possibleand that the description of the present invention in this embodiment isfor demonstrating the present memory access system, and is not intendedin a limiting or exclusive sense. For the purposes of this disclosure,the term "access" refers to any operation with a memory cell, including,but not limited to, reading from and writing to a memory cell.

The memory shown in the functional block diagram of FIG. 1 operatesaccording to well known principals. The eleven address lines shown tothe left of FIG. 1 are clocked into the row address buffer 150 by thesignal RAS (row address strobe) to select the row in the memory array tobe read or written. At a later time, the same eleven address lines areclocked into the column address buffer 160 by the signal CAS (columnaddress strobe) to select the column in the memory array to be read orwritten. The data lines shown in the right of FIG. 1 are bidirectionaldata ports used for both reading and writing data.

FIG. 2 shows a block diagram featuring some of the details of the senseamplifiers and I/O gating 110 block, the memory array 120 block and thecolumn decoder 130 block. The sense amplifiers and I/O gating 110include a number of write drivers 220, I/O sense amplifiers 230,passgates 210 and sense amplifiers 240. In FIG. 2 only a single writedriver 220, I/O sense amplifier 230, passgate 210 and sense amplifier240 are shown to demonstrate the operation of the circuit. Additionally,present memory cell circuits are differential in nature, however, thecircuit in FIG. 2 was simplified to a single ended circuit for purposesof demonstration. Furthermore, the memory array 120 as shown in FIG. 2depicts only a single memory cell 260 and access device 250 of aplurality of memory cells for the purposes of illustrating the operationof the circuit.

Column decoder 130 generates a column select signal 280 to enablepassgate 210 to pass signals between the access device 250 and the writedriver 220 and I/O sense amplifier 230.

In a write operation to memory cell 260, passgate 210 is activated andmemory cell 260 is written by write driver 220. After the memory cell260 is written, the column select signal 280 is switched to deactivatepassgate 210 and isolate the write driver 220 from the access device250, allowing write driver 220 to write to a different cell.

In a read operation, passgate 210 is activated and 1/0 sense amplifier230 receives the output of memory cell 260 as sensed and conditioned bysense amplifier 240. After the signal is received and I/O senseamplifier 230 is activated, the column select signal 280 is switched todeactivate passgate 210 and isolate the I/O sense amplifier 230 from theaccess device 250 and memory cell 260.

In a DRAM device, selection of the charge storage capacity of the memorycells is driven by a number of factors, including the amount ofcapacitance of the digit lines and the refresh rate of the device. Thememory cell capacitance is a factor in the access time of each memorycell, since each memory cell is charged by a voltage approximately equalto the chip supply voltage through a resistance. Theresistance-capacitance product provides the time constant of the typicalcell charging time, however, there are additional limitations in cellaccess times, such as charging of the digit lines and sense amplifiersetup times.

Assuming a fixed memory cell capacitance, the resistive portion of thememory cell charging circuit is the passgate 210 resistance in serieswith the internal resistance of the write driver 220, as shown in FIG.2. In most DRAM chip designs, time allocated for charging each memorycell is kept to a minimum to provide rapid access to the memory arrayand to minimize the time reserved for maintenance of the memory array,such as cell refresh operations.

If the column select signal 280 has a voltage magnitude of approximatelythe chip supply voltage, Vcc, then the passgate 210 will not pass theentire write voltage produced by write driver 220 during a writeoperation. The resulting logic one voltage passed will be approximatelythe chip supply voltage, Vcc, minus the threshold voltage, Vt, of thepassgate 210. The loss of the logic one voltage magnitude slows theWRITE/RFAD speed of the DRAM and decreases DRAM performance. If thelogic one voltage were a full Vcc value, then the charging of the memorycell during a write operation would be faster, due to a larger potentialacross the memory cell, and ohmic dissipation across the passgate 210would be minimized.

In FIG. 3, passgate 210 is an n-channel enhancement mode transistordevice and column boot circuitry 270 provides a booted voltage (Vb)which exceeds the chip supply voltage (Vcc). For the purposes of thisdescription, a booted voltage is any voltage exceeding the chip supplyvoltage (Vcc). The booted voltage, Vb, is applied to passgate 210 as acolumn select signal 280 so that the voltage passed through passgate 210may exceed the previous Vcc-Vt value. The general equation describingthe maximum voltage passed by the passgate 210 is approximately:

    maximum voltage passed=Vb-Vt.                              Eqn. 1

For example, for Vcc=5.0 volts, Vt=0.5 volts and Vb=5.4 volts, then themaximum voltage passed through the passgate 210 is 4.9 volts, for Vbapplied as a column select signal voltage. If Vcc=5.0 volts, Vt=0.5volts and Vb=5.8 volts, then the maximum voltage passed through thepassgate 210 is 5.3 volts, or more correctly, 5.0 volts if the voltageacross the passgate 210 does not exceed the power supply.

In one embodiment, the column select signal voltage is booted for awrite operation, but not for a read operation. For example, during awrite operation, Vb-Vt volts is transferred from write driver 220 tomemory cell 260 via access device 250. This increases the switchingspeed of the write operation, since any boot to the column select signalboth increases the voltage across the memory cell 250 and lowers theresistance of the passgate 210, decreasing the necessary charge time ofmemory cell 250.

A booted column select signal voltage is also beneficial during a readoperation, since the booted column select signal voltage applied topassgate 210 increases the amount of voltage sensed by I/O senseamplifier 230. This effectively increases the noise margin for signalsreceived from memory cell 260 and sense amplifier 240. The largervoltage passed by passgate 210 provides quicker reads from memory cell260, since the received voltage is larger. It also provides improvednoise margins, which in turn, may facilitate lower source voltages infuture DRAM designs and noisier signal environments from multilayer ordensely packed architectures.

Other combinations of selectively applying the booted column selectsignal for specific operations are possible without departing from thescope and spirit of the present invention.

Alternate embodiments employ a booted voltage produced elsewhere on theintegrated circuit by switching passgate 210 using the booted voltage asa column select signal 280.

Those skilled in the art will readily recognize that other boot voltagesources may be used without departing from the scope and spirit of thepresent invention. Furthermore, any boot voltage above Vcc improves theamount of signal passed by passgate 210. However, boot voltages musthave a voltage of at least the sum of the chip supply voltage, Vcc, andthe threshold voltage, Vt, of the passgate in order to pass the entirevoltage applied across the source and drain of passgate 210.

Although the embodiments described above relate to a DRAM device, thesame principals apply to any memory device having column access. FIG. 4shows a passgate 310 which separates memory cell access electronics 340and memory device 320. Column select circuit 330 sends a column selectsignal on signal line 350 to passgate 310 to perform a read or write ofmemory device 320. Column select circuit 330 includes a booted voltagesource to provide a booted column select signal to passgate 310. Themaximum voltage passed by passgate 210 is described by Eqn. 1, above.

For example, a static memory device may experience the same improvementsin switching and in noise margin as the DRAM embodiments describedabove. In this example, memory device 320 is an active memory cell usedin a static memory device. Therefore, several embodiments exist whichare not presented here and the present system may be applied to anymemory device having columns of memory cells to access.

Alternate embodiments employing p-channel devices require a negativeboot voltage, and those skilled in the art will readily recognize thatsuch designs do not depart from the scope and spirit of the presentinvention.

Although specific embodiments have been illustrated and described hereinfor purposes of description of the preferred embodiment, it will beappreciated by those of ordinary skill in the art that a wide variety ofalternate and/or equivalent implementations calculated to achieve thesame purposes may be substituted for the specific embodiment shown anddescribed without departing from the scope of the present invention. Forexample, particular voltages are described in this specification,however the voltages may be increased or decreased and circuitconfigurations may vary without departing from the scope and spirit ofthe present invention.

Those with skill in the electrical, computer, and telecommunicationsarts will readily appreciate that the present invention may beimplemented in a very wide variety of embodiments. This application isintended to cover any adaptations or variations of the preferredembodiment discussed herein. Therefore, it is manifestly intended thatthis invention be limited only by the claims and the equivalentsthereof.

What is claimed is:
 1. A column select system for an integrated circuitmemory having an external chip supply voltage, said memory including aplurality of memory cells for storing information, said column selectsystem comprising:an access device includinga gate connected to a wordline; a drain connected to a digit line; and a source connected to oneof the memory cells; a passgate comprising an n-channel transistordevice includinga first port connected to the digit line; a second portconnected to a write driver and an I/O sense amplifier, the write driverproviding a voltage corresponding to the chip supply voltage; andaswitchable gate, and a column driver connected to the switchable gate ofthe passgate, the column driver comprising column boot circuitry forproducing an internal booted column select signal having a voltage thatis at least the external chip supply voltage plus a threshold voltage ofthe passgate for enabling the passgate to transfer voltagesapproximately equal to the external chip supply voltage to said onememory cell.
 2. A column select system for an integrated circuit memoryhaving an external chip supply voltage, said memory including aplurality of memory cells for storing information, the plurality ofmemory cells configured in a plurality of columns, said column selectsystem comprising:a passgate comprising an n-channel transistor deviceincludinga first port coupled to at least one of said memory cells; asecond port connected to memory cell access electronics having writingand reading voltages approximately equal to the chip supply voltage; anda switchable gate; and a column select circuit connected to theswitchable gate of the passgate, the column select circuit comprisingcolumn boot circuitry for producing an internal booted column selectsignal having a voltage that is at least the external chip supplyvoltage plus a threshold voltage of the passgate for enabling thepassgate to transfer voltages approximately equal to the external chipsupply voltage between the cell access electronics and said one memorycell.
 3. A method for column access for an integrated circuit memoryhaving an external chip supply voltage, said method comprising the stepsof:generating an internal booted column select signal having a voltagethat is at least than the external chip supply voltage plus a thresholdvoltage of an n-channel transistor device acting as a passgate; andapplying the internal booted column select signal to the passgate forturning the passgate on to connect a memory cell in a memory array tomemory cell access electronics to transfer voltages between the memorycell access electronics and the memory cell which are approximatelyequal to the external chip supply voltage.
 4. A column select system foran integrated circuit memory having an external chip supply voltage anda base voltage, said memory including a plurality of memory cells forstoring information, said column select system comprising:an accessdevice includinga gate connected to a word line; a drain connected to adigit line; and a source connected to one of the memory cells; apassgate comprising a p-channel transistor device includinga first portconnected to the digit line; a second port connected to a write driverand an I/O sense amplifier, the write driver providing a voltagecorresponding to the chip supply voltage; anda switchable gate, and acolumn driver connected to the switchable gate of the passgate, thecolumn driver comprising column boot circuitry for producing an internalbooted column select signal over a full voltage range from the basevoltage minus a threshold voltage of the passgate to the external chipsupply voltage for enabling the passgate to transfer voltagesapproximately equal to the external chip supply voltage to said onememory cell.
 5. A column select system for an integrated circuit memoryhaving an external chip supply voltage and a base voltage, said memoryincluding a plurality of memory cells for storing information, theplurality of memory cells configured in a plurality of columns, saidcolumn select system comprising:a passgate comprising a p-channeltransistor device includinga first port coupled to at least one of saidmemory cells; a second port connected to memory cell access electronicshaving writing and reading voltages approximately equal to the chipsupply voltage; and a switchable gate; and a column select circuitconnected to the switchable gate of the passgate, the column selectcircuit comprising column boot circuitry for producing an internalbooted column select signal over a full voltage range from the basevoltage minus a threshold voltage of the passgate to the external chipsupply voltage for enabling the passgate to transfer voltagesapproximately equal to the external chip supply voltage between the cellaccess electronics and said one memory cell.
 6. A method for columnaccess for an integrated circuit memory having an external chip supplyvoltage and a base voltage, said method comprising the stepsof:generating an internal booted column select signal over a fullvoltage range from the base voltage minus a threshold voltage of ap-channel transistor device acting as a passgate to the external chipsupply voltage; and applying the internal booted column select signal tothe passgate for turning the passgate on to connect a memory cell in amemory array to memory cell access electronics to transfer voltagesbetween the memory cell access electronics and the memory cell which areapproximately equal to the external chip supply voltage.